1. Field of the Invention
The present invention relates to a semiconductor wafer used as a substrate for various kinds of semiconductor device and to a manufacturing method for the semiconductor wafer.
2. Description of the Prior Art
Usually, an epitaxial wafer is often doped with impurity in high concentration to provide a p-type or an n-type conductive element.
For example, when a conductive element is of p-type, boron(B) is doped as the impurity into a silicon wafer in high concentration, and when it is of n-type, phosphorus (P), antimony(Sb), arsenic (As), etc. are doped as the same.
When such doped wafer is heated at high temperature of 1000.degree. to 1200.degree. C. to grow silicon epitaxially thereon, it is well-known that boron, phosphorus, antimony, arsenic or the like jump out from the substrate wafer to enter the epitaxial growth layer to thereby produce the so-called auto-doping phenomenon, resulting in that the electrical characteristic is changed.
Jumping out of boron, phosphorus, antimony or arsenic from the wafer is suppressed at the main surface thereof because of forming the epitaxial growth layer, whereby such element jumps out mainly from the peripheral surface and the rear surface (opposite to the main surface). Hence, in order to prevent jumping out of much element from occurring at this part, a method has hitherto been adopted which forms a film of SiO.sub.2 and/or Si.sub.3 N.sub.4 or the like as a blocking film thereat.
FIG. 1 is an illustration of the process of forming the epitaxial layer at the conventional wafer. As shown in FIG. 1-(a), for preventing a peripheral edge portion of a disclike semiconductor substrate wafer from being chipped off when it is handled, it is chamfered at both surfaces to form tapered faces 1a and 1b and a portion 1c arcuated in section therebetween and after a damaged layer is removed by chemical etching, the blocking film is formed over the entire surface of the substrate wafer 1 by an atmospheric pressure CVD method or a thermal oxidation method. FIG. 1-(b) shows the wafer on which one or two layers of blocking film of SiO.sub.2 and/or Si.sub.3 N.sub.4 are laminated by the atmospheric pressure CVD method. FIG. 1-(b') shows the wafer on which the aforesaid layers are laminated by the thermal oxidation method. The blocking film, when formed by the atmospheric pressure CVD method, is smaller in thickness at the main surface (the upper surface in the drawing) of the substrate wafer 1 as shown in FIG. 1(b), and when formed by the thermal oxidation method, is substantially uniform in thickness through the entire surface as shown in FIG. 1(b').
Upon completing forming the blocking film 2, the substrate wafer 1 is subjected at the main surface side thereof to polish, the blocking film 2 formed on the main surface is removed by lapping, and the main surface is finished to be a mirror. Thus, as shown in FIG. 1-(c), a wafer 10, which is covered with the blocking film 2 on the entire rear surface and the peripheral surface except for about half a portion of the tapered face 1a at the main surface side, is obtained. Such wafer 10 is provided at the main surface thereof with an epitaxial layer 3 as shown in FIG. 1-(d).
When the epitaxial growth of silicon is applied onto the main surface of the water 10 having the blocking film 2 formed thereon as abovementioned, since the blocking film 2 is formed on the peripheral surface and the rear surface of the same, the auto-doping from the substrate wafer 1 toward the epitaxial layer 3 is remarkably suppressed during the process of forming the epitaxial layer 3 on the main surface of the substrate wafer 1, thereby expecting an improvement in quality of the epitaxial layer 3 itself.
FIG. 2 is a graph showing an extent of auto-doping when the blocking film is formed (shown by solid line) and that when it is not formed (shown by the broken line) as the results of testing the spreading resistance (SR). A depth from the epitaxial layer surface is taken in the axis of abscissa and the impurity concentration (logarithmic scale, optional unit) is taken in the axis of ordinate, whereby it is apparent that the impurity concentration at the epitaxial layer is largely reduced by forming the blocking film.
On the other hand, as seen from FIG. 1-(d), silicon included in reaction gas is formed in many granulated particles polysilicon 3a on the blocking film 2, especially on the peripheral surface of the substrate wafer 1, during the process of epitaxial growth on the main surface of the same. The silicon particles 3a drop from the surface of the blocking film and attach to the surface of the epitaxial layer 3 or the like during the process of producing a semiconductor device (product), thereby causing contamination thereon, and creating a problem in that the yield of product lowers.
FIG. 3 is a microphotograph showing an outside appearance taken on the line III--III in FIG. 1-(d) and FIG. 4 is one taken on the line IV--IV in FIG. 1-(d). As seen from them, many silicon particles are formed on the peripheral surface of the blocking film.